Bone Anchor Assemblies and Methods With Improved Locking

ABSTRACT

Various exemplary methods and devices are provided for fixing bone anchors to bone. In general, the methods and devices can allow for a bone anchor to be fixed to a bone at a desired angle to a receiver member. In an exemplary embodiment, a bone anchor assembly is provided that includes a bone anchor configured to engage bone, a receiver member that seats a spherical head of the bone anchor and a spinal fixation element, a compression member seated within the receiver member, proximally of the head of the bone anchor, and a closure mechanism seated within the receiver member, proximally of the compression member. One or more of the receiver member, the compression member, and the closure mechanism can have gripping features thereon configured to secure the bone anchor and/or the spinal fixation element within the receiver member, thereby reducing a risk of slippage of the bone anchor and/or the spinal fixation element with respect to the receiver member.

RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. application Ser. No. 13/826,161, filed Mar. 14, 2013, entitled “Bone Anchor Assemblies and Methods With Improved Locking,” the entirety of which is incorporated herein by reference.

FIELD

The present invention relates to methods and devices for correcting a spine, and in particular to bone anchor assemblies and methods of using the same.

BACKGROUND

Bone anchors may be used in orthopedic surgery to fix bone during healing or during a fusion process. In spinal surgery, bone anchors may be used with spinal fixation elements, such as spinal rods, to stabilize multiple vertebrae either rigidly, in which no relative motion between the vertebrae is desired, or dynamically, in which limited, controlled motion between the vertebrae is desired. Fixation elements can help to support the spine in a desired alignment, for example by defining a shape towards which a deformed spine is to be corrected. Attaching the vertebrae to the fixation element causes vertebrae which are out of position to be drawn towards the fixation element, so that they can then be retained in a correct alignment against forces imposed by soft tissue tending to revert the configuration of the spine towards the deformed shape. Correction of the spinal deformation can involve application to the vertebrae of translational forces, torsional forces, or combinations thereof to cause vertebrae to translate and/or rotate.

Traditional bone anchor assemblies include a bone anchor having a rod-receiving member formed thereon or coupled thereto for seating a spinal fixation rod. A compression member disposed distally of the spinal fixation rod and proximally of the bone anchor can be provided for locking the bone anchor at a fixed angular orientation relative to the rod-receiving member. A closure mechanism disposed proximally of the spinal fixation rod and can be provided to lock both the bone anchor and the spinal fixation element within the rod-receiving member. While the use of a compression member in combination with a closure mechanism can be an effective means to secure the assembly, traditional compression members and closure mechanisms can be prone to slip relative to the bone anchor, the rod-receiving member, and/or relative to one another. Slippage of either of these components can cause the bone anchor and/or the spinal fixation rod to move from a desired orientation within the rod-receiving member and can therefore compromise the effectiveness of the bone anchor assembly for correcting spinal deformities.

Accordingly, there remains a need for improved methods and devices for bone anchor fixation.

SUMMARY

The present invention generally provides methods and devices for bone anchor fixation. In one aspect, a bone anchor assembly is provided that can include a bone anchor having a proximal head portion and a distal shank portion, a receiver member having a polyaxial seat formed therein for polyaxially seating the head portion of the bone anchor, a closure mechanism matable with the receiver member to lock the bone anchor within the receiver member, and a compression member. The compression member can be configured to be disposed within the receiver member between the bone anchor and the closure mechanism and can have a proximal end configured to seat a distal end of the closure mechanism. The proximal end of the compression member can have a shape that causes the compression member to move radially inward when a distally-directed force is applied thereto by the closure mechanism that exceeds a threshold force.

The compression member can have a variety of configurations and can be configured to interact with the closure mechanism in a variety of ways. In one aspect, the distal end of the closure mechanism can be seated in the proximal end of the compression member such that the compression member is freely rotatable but does not move radially inward or outward unless a force greater than the threshold force is applied thereto. In another aspect, the proximal end of the compression member can have a convex shape that corresponds to a concave shape of the distal end of the closure mechanism. In some embodiments, the closure mechanism can include at least one threaded member.

In other embodiments, the proximal end of the compression member can have a height that decreases radially outward. The compression member can additionally or alternatively have a distal end that abuts against the proximal head of the bone anchor, and the distal end can include a distally-extending skirt formed around at least a portion of an outer perimeter thereof such that the skirt extends distally beyond a proximal-most end of the head portion of the bone anchor when the compression member and the bone anchor are disposed within the receiver member. In one embodiment, the skirt can extend all the way around a circumference of the head portion of the bone anchor. To define the skirt, an outer portion of a distal facing end surface of the compression member can extend at an angle relative to an inner portion of the distal facing end surface that is planar and extends substantially perpendicularly to a central longitudinal axis of the compression member.

In another aspect, a bone anchor assembly is provided that can include a bone anchor having a proximal head portion and a distal shank portion, a receiver member having a polyaxial seat formed in a distal portion thereof for polyaxially seating the head portion of the bone anchor, and a closure mechanism. The closure mechanism can have an outer member and an inner member. The outer member can have outer threads configured to threadably mate with threads formed in the receiver member to thereby lock the proximal head of the bone anchor with respect to the receiver member. The outer member can also have a central opening with inner threads formed therein. The inner member can have outer threads formed therearound for threadably mating with the inner threads formed in the outer member and can be configured to lock a spinal fixation element within the receiver member. At least one of the outer threads on the inner member and the inner threads on the outer member can be configured to provide an interference fit when the inner member is threadably mated to the outer member.

The threads of the inner and outer members can be configured in a variety of ways. In one aspect, at least a portion of the outer threads on the inner member can have a pitch that is different from a pitch of the inner threads on the outer member. In another aspect, at least one of the outer threads on the inner member and the inner threads on the outer member can include a mechanical deformation formed thereon and configured to create the interference fit when the inner and outer members are threadably mated. The mechanical deformation can comprise a surface protrusion formed on at least one of the outer threads on the inner member and the inner threads on the outer member and configured to extend into a surface of an adjacent thread.

The bone anchor assembly can further include a compression member that can be configured to be disposed within the receiver member between the bone anchor and the closure mechanism. Additionally or alternatively, each of the bone anchor assembles described herein can include an expandable clip that can be seated around a head of the bone anchor and/or within a groove formed in the receiver member. The expandable clip can be configured to apply a frictional drag force to the head portion of the bone anchor.

The present invention further provides devices, systems, and methods as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1A is a perspective view of a prior art bone anchor assembly;

FIG. 1B is an exploded view of the bone anchor assembly of FIG. 1A;

FIG. 1C is a cross-sectional view of a portion of the bone anchor assembly of FIG. 1A;

FIG. 1D is a top view of the bone anchor assembly of FIG. 1A;

FIG. 1E is a cross-sectional view of the bone anchor assembly of FIG. 1A;

FIG. 2 is a cross-sectional view of one embodiment of a compression member for use with a bone anchor assembly;

FIG. 3 is a cross-sectional view of another embodiment of a compression member for use with a bone anchor assembly;

FIG. 4 is a cross-sectional view of another embodiment of a compression member for use with a bone anchor assembly;

FIG. 5 is a cross-sectional view of another embodiment of a compression member for use with a bone anchor assembly;

FIG. 6 is a cross-sectional view of another embodiment of a compression member for use with a bone anchor assembly;

FIG. 7 is a cross-sectional view of another embodiment of a compression member for use with a bone anchor assembly;

FIG. 8 is a cross-sectional view of another embodiment of a compression member for use with a bone anchor assembly;

FIG. 9 is a cross-sectional view of a receiver member for use with a bone anchor assembly;

FIG. 10 is a partial cross-sectional view of the receiver member of FIG. 9;

FIG. 11 is a partial cross-sectional view of the receiver member of FIG. 9 having a bone anchor seated therein;

FIG. 12 is a cross-sectional view of an exemplary embodiment of a closure mechanism for use with a bone anchor assembly;

FIG. 13 is a cross-sectional view of a portion of the closure mechanism of FIG. 12;

FIG. 14 is a cross-sectional view of another exemplary embodiment of a closure mechanism for use with a bone anchor assembly;

FIG. 15 is a cross-sectional view of a portion of the closure mechanism of FIG. 14;

FIG. 16 is a perspective view of an inner set screw of the closure mechanism of FIG. 14;

FIG. 17 is a cross-sectional view of a bone anchor assembly showing the compression member of FIG. 5 oriented in a first orientation relative to the receiver member of FIG. 9;

FIG. 18 is a cross-sectional view of another a bone anchor assembly showing the compression member of FIG. 5 oriented in a second orientation relative to the receiver member of FIG. 9; and

FIG. 19 is a cross-sectional view of the compression member of FIG. 8 that is mated to an outer screw of the closure mechanism of FIG. 14.

DETAILED DESCRIPTION

Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the devices and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the devices and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present invention is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present invention.

Various exemplary methods and devices are provided for fixing bone anchors to bone. In general, the methods and devices can allow for a bone anchor to be fixed to a bone at a desired angle relative to a receiver member. In an exemplary embodiment, a bone anchor assembly is provided that includes a bone anchor configured to engage bone, a receiver member that polyaxially seats a spherical head of the bone anchor, and a compression member for securing the receiver member at a fixed angle with respect to the bone anchor. The compression member can be seated within the receiver member, proximally of the head of the bone anchor, and can have a distal end with one or more gripping features thereon configured to grip the head of the bone anchor, even where the bone anchor is oriented at an angle to a longitudinal axis of the compression member. By way of non-limiting example, the one or more gripping features can be configured to create at least one of a line contact and/or a band contact with the head of the bone anchor, thus providing a firm grip on the head of the bone anchor and reducing a risk of slippage of the bone anchor with respect to the receiver member.

FIGS. 1A-1E illustrate a prior art bone anchor assembly 10 including a bone anchor 12, a receiver member 14 for receiving a spinal fixation element, such as a spinal rod 22, to be coupled to the bone anchor 12, and a closure mechanism 16 to capture a spinal fixation element within the receiver member 14 and fix the spinal fixation element with respect to the receiver member 14. The bone anchor 12 includes a proximal head 18 and a distal shaft 20 configured to engage bone. The receiver member 14 has a proximal end 26 having a pair of spaced apart arms 28A, 28B defining a recess 30 therebetween and a distal end 32 having an inner surface 35 for polyaxially seating the proximal head 18 of the bone anchor 12 and distal end surface 34 defining an opening through which at least a portion of the bone anchor 12 extends. The closure mechanism 16 can be positionable between and can engage the arms 28A, 28B to capture a spinal fixation element, e.g., a spinal rod 22, within the receiver member 14 and fix the spinal fixation element with respect to the receiver member 14.

The proximal head 18 of the bone anchor 12 is generally in the shape of a truncated sphere having a planar proximal surface 36 and an approximately spherically-shaped distal surface 38. The illustrated bone anchor assembly is a polyaxial bone anchor designed for posterior implantation in the pedicle or lateral mass of a vertebra. The proximal head 18 of the bone anchor 12 engages the distal end 32 of the receiver member 14 in a ball and socket like arrangement in which the proximal head 18 the distal shaft 20 can pivot relative to the receiver member 14. The distal surface 38 of the proximal head 18 of the bone anchor 12 and a mating surface within the distal end 32 of the receiver member 14 can have any shape that facilitates this arrangement, including, for example, spherical (as illustrated), toroidal, conical, frustoconical, and any combinations of these shapes.

The distal shaft 20 of the bone anchor 12 can be configured to engage bone and, in the illustrated embodiment, includes an external bone engaging thread 40. The thread form for the distal shaft 20, including the number of threads, the pitch, the major and minor diameters, and the thread shape, can be selected to facilitate connection with bone. Exemplary thread forms are disclosed in U.S. Patent Application Publication No. 2011/0288599, filed on May 18, 2011, and in U.S. Provisional Patent Application Ser. No. 61/527,389, filed Aug. 25, 2011, both of which are incorporated herein by reference. The distal shaft 20 can also include other structures for engaging bone, including a hook. The distal shaft 20 of the bone anchor 12 can be cannulated, having a central passage or cannula extending the length of the bone anchor to facilitate delivery of the bone anchor over a guide wire in, for example, minimally-invasive procedures. Other components of the bone anchor assembly, including, for example, the closure member 16, the receiver member 14, and the compression member 60 (discussed below) can be cannulated or otherwise have an opening to permit delivery over a guide wire or to permit the insertion of a driver instrument to manipulate the bone anchor. The distal shaft 20 can also include one or more sidewall openings or fenestrations that communicate with the cannula to permit bone in-growth or to permit the dispensing of bone cement or other materials through the bone anchor 12. The sidewall openings can extend radially from the cannula through the sidewall of the distal shaft 20. Exemplary systems for delivering bone cement to the bone anchor assembly 10 and alternative bone anchor configurations for facilitating cement delivery are described in U.S. Patent Application Publication No. 2010/0114174, filed on Oct. 29, 2009, which is hereby incorporated herein by reference. The distal shaft 20 of the bone anchor 12 can also be coated with materials to permit bone growth, such as, for example, hydroxyl apatite, and the bone anchor assembly 10 can be coated partially or entirely with anti-infective materials, such as, for example, tryclosan.

The proximal end 26 of the receiver member 14 includes a pair of spaced apart arms 28A, 28B defining a U-shaped recess 30 therebetween for receiving a spinal fixation element, e.g., a spinal rod 22. Each of the arms 28A, 28B can extend from the distal end 32 of the receiver member 14 to a free end. The outer surfaces of each of the arms 28A, 28B can include a feature, such as a recess, dimple, notch, projection, or the like, to facilitate connection of the receiver member 14 to instruments. For example, the outer surface of each arm 28A, 28B can include an arcuate groove at the respective free end of the arms. Such grooves are described in more detail in U.S. Pat. No. 7,179,261, issued on Feb. 20, 2007, which is hereby incorporated herein by reference. At least a portion of the proximal end surface 48 of the receiver member 12 defines a plane Y. The receiver member 14 has a central longitudinal axis L.

The distal end 32 of the receiver member 14 includes a distal end surface 34 which is generally annular in shape defining a circular opening through which at least a portion of the bone anchor 12 extends. For example, the distal shaft 20 of the bone anchor 12 can extend through the opening. At least a portion of the distal end surface 34 defines a plane X.

The bone anchor 12 can be selectively fixed relative to the receiver member 14. Prior to fixation, the bone anchor 12 is movable relative to the receiver member 14 within a cone of angulation generally defined by the geometry of the distal end 32 of the receiver member and the proximal head 18 of the bone anchor 12. The illustrated bone anchor is a favored-angle polyaxial screw in which the cone of angulation is biased in one direction. In this manner, the bone anchor 12 is movable relative to the receiver member 14 in at least a first direction, indicated by arrow A in FIG. 1E, at a first angle C relative to the central longitudinal axis L of the receiver member 14. The bone anchor 12 is also movable in at least a second direction, indicated by arrow B in FIG. 1E, at a second angle D relative to the longitudinal axis L. The first angle C is greater than the second angle D and, thus, the shaft 20 of the bone anchor 12 is movable more in the direction indicated by arrow A than in the direction indicated by arrow B. The distal shaft 20 of the bone anchor 12 defines a neutral axis 48 with respect to the receiver member 14. The neutral axis 48 can be perpendicular to the plane X defined by the distal end surface 34 and intersects the center point of the opening in the distal end surface 34 through which the distal shaft 20 of the bone anchor 12 extends. The neutral axis 48 can be oriented at an angle to the central longitudinal axis L of the receiver member 14. The plane Y defined by at least a portion of the proximal end surface 48 of the receiver member 14 intersects the plane X defined by at least a portion of the distal end surface 34 of the receiver member 12. The proximal end 26 of the receiver member 14 can include a proximal first bore 50 coaxial with a first central longitudinal axis N (which is coincident with longitudinal axis L) and a distal second bore 52 coaxial with a second central longitudinal axis M (which is coincident with the neutral axis 48) and the first central longitudinal axis N and second central longitudinal axis M can intersect one another. The angle between the plane X and the plane Y and the angle between the axis L and the axis M can be selected to provide the desired degree of biased angulation. Examples of favored angled polyaxial screws are described in more detail in U.S. Pat. No. 6,974,460, issued on Dec. 13, 2005, and in U.S. Pat. No. 6,736,820, issued on May 18, 2004, both of which are hereby incorporated herein by reference. Alternatively, the bone anchor assembly can be a conventional (non-biased) polyaxial screw in which the bone anchor pivots in the same amount in every direction and has a neutral axis that is coincident with the central longitudinal axis L of the receiver member.

The spinal fixation element, e.g., the spinal rod 22, can either directly contact the proximal head 18 of the bone anchor 12 or can contact an intermediate element, e.g., a compression member 60. The compression member 60 can be positioned within the receiver member 14 and interposed between the spinal rod 22 and the proximal head 18 of the bone anchor 12 to compress the distal outer surface 38 of the proximal head 18 into direct, fixed engagement with the distal inner surface of the receiver member 14. A proximal portion of the compression member 60 can include a pair of spaced apart arms 62A and 62B defining a U-shaped seat 64 for receiving the spinal rod 22. A distal portion of the compression member 60 can include a sidewall having an inner cylindrical surface 67 that is connected to an outer cylindrical surface 68 by a distal-facing surface 66.

At least a portion of the distal surface 66 of the compression member 60 can be shaped as a negative of the proximal portion 18 of the bone anchor 20, against which the distal surface 66 abuts when the compression member 60 is fully inserted into the receiver member 14. Thus, when the shaft 20 of the bone anchor 12 is oriented along the longitudinal axis L, the contact area between the distal surface 66 of the compression member 60 and the proximal head 18 is maximized. Where the angle of the shaft 20 with respect to the longitudinal axis L is not zero, however, the contact area between the distal surface 66 of the compression member 60 and the head 18 can be reduced, thus increasing a risk of slippage of the bone anchor 12 with respect to the receiver member 14.

As best seen in FIG. 1B, the compression member 60 is configured to slide freely along the longitudinal axis L within the recess 30 of the receiver member 14. To secure the compression member 60 within the receiver member 14, the compression member 60 can be configured to mate with the receiver member, for example by mechanically deforming a portion of the compression member 60 against the receiver member 14. In the illustrated embodiment, opposing bores formed in the arms 62A, 62B of the compression member 60 are aligned with bores formed in the arms 62A, 62B of the receiver member 14, such that opposing pins can be inserted through the passageways defined by the bores to compress or “swage” the compression member 60 against the receiver member 14. The swaging process can prevent subsequent removal of the compression member 60 from the receiver member 14.

A number of gripping features can be used in addition to, or as an alternative to, the aforementioned gripping features of the compression member 60. By way of non-limiting example, FIG. 1C illustrates a ring member, e.g., a snap ring 120, for frictionally engaging the head of the bone anchor. As explained in U.S. application Ser. No. 13/657,486, filed on Oct. 22, 2012 and hereby incorporated by reference in its entirety, the snap ring 120 can have a variety of configurations, shapes, and sizes, but it should be adapted to expand to fit around at least a portion of the head 18 of the bone anchor 12 and to thereby exert a frictional drag force on the head 18. As shown, the snap ring 120 is in the shape of a loop with an opening formed therein that allows the diameter d of the snap ring 120 to expand to fit around a portion of the spherical head 18 of the bone anchor 12. A groove 122 formed within the recess 30 of the receiver member 14 maintains the snap ring 120 at a particular location with respect to the spherical head 18 such that the snap ring 120 is expanded around the head 18. By way of non-limiting example, the groove 122 can be configured to maintain the snap ring 120 at a position just proximal to a center line of the head 18.

The proximal end 26 of the receiver member 14 can be configured to receive a closure mechanism 16 positionable between and engaging the arms 28A, 28B of the receiver member 14. The closure mechanism 16 can be configured to capture a spinal fixation element, e.g., a spinal rod 22, within the receiver member 14, to fix the spinal rod 22 relative to the receiver member 14, and to fix the bone anchor 12 relative to the receiver member 14. The closure mechanism 16 can be a single set screw having an outer thread for engaging an inner thread 42 provided on the arms 28A, 28B of the receiver member 14. In the illustrated embodiment, however, the closure mechanism 16 comprises an outer set screw 70 positionable between and engaging the arms 28A, 28B of the receiver member 14 and an inner set screw 72 positionable within the outer set screw 70. The outer set screw 70 is operable to act on the compression member 60 to fix the bone anchor 12 relative to the receiver member 14. The inner set screw 72 is operable to act on the spinal rod 22 to fix the spinal rod 22 relative to the receiver member 14. In this manner, the closure mechanism 16 permits the bone anchor 12 to be fixed relative to the receiver member 14 independently of the spinal rod 22 being fixed to the receiver member 14. In particular, the outer set screw 70 can engage the proximal end surfaces of the arms 62A, 62B of the compression member 60 to force the distal-facing surface 66 of the compression member 60 into contact with the proximal head 18 of bone anchor 12, which in turn forces the distal surface 38 of the proximal head 18 into fixed engagement with the distal inner surface of the receiver member 14. The inner set screw 72 can engage the spinal rod 22 to force the spinal rod 22 into fixed engagement with the rod seat 64 of the compression member 60.

The outer set screw 70 includes a first outer thread 74 for engaging a complementary inner thread 42 on the arms 28A, 28B of the receiver member 14. The outer set screw 74 includes a central passage 96 from a top surface 98 of the outer set screw 74 to a bottom surface 100 of the outer set screw 74 for receiving the inner set screw 72. The central passage 96 can includes an inner thread 102 for engaging a complementary outer thread 104 on the inner set screw 72. The thread form for the inner thread 102 and the outer thread 104, including the number of threads, the pitch, major and minor diameter, and thread shape, can be selected to facilitate connection between the components and transfer of the desired axial tightening force. The top surface 98 of the outer set screw 74 can have one or more drive features to facilitate rotation and advancement of the outer set screw 74 relative to the receiver member 14. The illustrated outer set screw 74 includes drive features in the form of a plurality of cut-outs 106 spaced-apart about the perimeter of the top surface 98. The inner set screw 104 can include drive features for receiving an instrument to rotate and advance the inner set screw 72 relative to the outer set screw 74. The illustrated inner set screw 104 includes drive features in the form of a central passage 108 having a plurality of spaced apart, longitudinally oriented cut-outs for engaging complementary features on an instrument.

The bone anchor assembly 10 can be used with a spinal fixation element such as rigid spinal rod 22. The various components of the bone anchor assemblies disclosed herein, as well as the spinal rod 22, can be constructed from various materials, including titanium, titanium alloys, stainless steel, cobalt chrome, PEEK, or other materials suitable for rigid fixation. In other embodiments, the spinal fixation element can be a dynamic stabilization member that allows controlled mobility between the instrumented vertebrae.

In use, bone can be prepared to receive the bone anchor assembly 10, generally by drilling a hole in the bone which is sized appropriately to receive the bone anchor 12. If not already completed, the bone anchor assembly 10 can be assembled, which can include assembling the bone anchor 12 and the receiver member 14, so that the distal shaft 20 extends through the opening in the distal end 32 of the receiver member 14 and the proximal head 18 of the bone anchor 12 is received in the distal end 32 of the receiver member 14. A driver tool can be fitted with the bone anchor 12 to drive the bone anchor 12 into the prepared hole in the bone. The compression member 60 can be positioned within the receiver member 14 such that the arms 62A, 62B of the compression member are aligned with the arms 28A, 28B of the receiver member 14 and the lower surface of the compression member 14 is in contact with the proximal head 18 of the bone anchor 12. A spinal fixation element, e.g., the spinal rod 22, can be located in the recess 30 of the receiver member 14. The closure mechanism 16 can be engaged with the inner thread 42 provided on the arms 28A, 28B of the receiver member 14. A torsional force can be applied to the outer set screw 70 to move it within the recess 30 using a tool which can engage the plurality of cut-outs 106 in the upper facing surface of the outer set screw 70, so as to force the compression member 60 onto the proximal head 18 of the bone anchor 12. Torsional forces can then be applied to the inner set screw 72 to move it relative to the outer set screw 70 so that it contacts the spinal rod 22 and can, for example, fix the spinal rod 22 relative to the receiver member 14 and the bone anchor 12.

One or more embodiments of inventive bone anchor assemblies are described below. Except as indicated below, the structure, operation, and use of these embodiments is similar or identical to that of the bone anchor assembly 10 described above. Accordingly, a detailed description of said structure, operation, and use is omitted here for the sake of brevity. FIGS. 2-8 show various embodiments of compression members similar to the compression member 60 shown in FIG. 1B and having gripping features on a distal end thereof for gripping a head of a bone anchor with greater friction as compared with a distal end formed as a negative of the head portion of the bone anchor. The compression members shown in FIGS. 2-8 can be used with the bone anchor assembly 10 shown in FIGS. 1A-1E, or with various other bone anchor assemblies known in the art. FIGS. 9-11 show various embodiments of receiver member similar to the receiver member 14 shown in FIG. 1B and having gripping features formed within a distal recess therein for gripping the underside of a head of a bone anchor with greater friction as compared with a recess in a receiver formed as a negative of the head portion of the bone anchor. The receiver members shown in FIGS. 9-11 can be used with the bone anchor assembly shown in FIGS. 1A-1E, or with various other bone anchor assemblies known in the art. FIGS. 12-16 show various embodiments of a closure mechanism similar to the closure mechanism 16 shown in FIG. 1B and comprising inner and outer members. The closure mechanisms of FIGS. 12-16 can be used with the bone anchor assembly 10 shown in FIGS. 1A-1E, or with various other bone anchor assemblies known in the art.

FIG. 2 illustrates an exemplary embodiment of a compression member for use with a bone anchor assembly of the type described above. The illustrated compression member 160 has a distal end that is configured to grip a proximal head of a bone anchor to secure the head at a desired angle with respect to a receiver member (not shown) when the compression member is locked within the receiver member, e.g., when a closure mechanism is applied to the receiver member to lock the bone anchor in a fixed position relative to the receiver member. Although the distal end of the compression member 160 can be configured to grip the head of a bone anchor in a variety of ways, in the illustrated embodiment, the compression member 160 includes a planar distal-facing surface 166 that extends between an inner cylindrical surface 167 and an outer cylindrical surface 168. The distal-facing surface 166 can extend in a plane perpendicular to the longitudinal axis L₁ of the compression member 160, and the surface can have a generally circular shape with an inner circular corner or edge 161. When the compression member 160 is secured locked within a receiver member, the edge 161 can create a ring-shaped line contact with the head of the bone anchor to substantially prevent movement of the head and to ensure that the bone anchor remains at a fixed angle with respect to the receiver member. Such line contact can be particularly advantageous with favored-angle bone anchors in which the bone anchor is at an extreme angle relative to the receiver member. As explained above with respect to FIGS. 1A-1E, the compression member 160 can be locked within the receiver member by applying a closure mechanism, e.g., inner and/or outer set screws, to the receiver member. The closure mechanism can apply a distally directed force to a spinal fixation element, e.g., a spinal rod, seated within the receiver member, and the spinal rod in turn can apply a distally directed force to the compression member 160 to cause the compression member 160 to press down on and engage the head of the bone anchor. In other embodiments, the closure mechanism can be configured to provide a force directly to the compression member 160 to lock the head of the bone anchor in a fixed position relative to the receiver member without locking the spinal fixation rod within the receiver member. For example, the closure mechanism can include inner and outer set screws, with the inner set screw locking the rod and the outer set screw locking the compression member and thus the bone anchor. A person skilled in the art will appreciate that a variety of locking techniques can be utilized.

The configuration of the edge 161 can vary, and in one embodiment the edge 161 can have a radius of curvature corresponding to a radius of curvature of the head of a bone anchor to be used therewith. In some embodiments, a largest diameter of the edge 161 can be smaller than a diameter of the head where the edge 161 grips the head to create an interference fit between the compression member 160 and the head. In the illustrated embodiment, the edge 161, and thus the ring-shaped line contact, extends in a plane that is substantially perpendicular to a longitudinal axis L₁ of the compression member 160, but it will be appreciated by a person skilled in the art that the line contact can be formed in a plane that is oriented at any angle to the longitudinal axis L₁ of the compression member 160.

FIGS. 3-8 illustrate additional embodiments of compression members with distal-facing surfaces having various shapes and various gripping features thereon configured to grip a head of a bone anchor when the compression member is in a locked configuration. The compression members of FIGS. 3-8 can generally be configured and used similar to the compression member 160 of FIG. 2. Additionally, like-named elements and like-illustrated elements of the compression member 160 and of the other compression members discussed herein can be configured and used similar to one another.

In the embodiment of FIG. 3, the compression member 260 has a sloped distal-facing surface 266 that extends between an inner cylindrical surface 267 and an outer cylindrical surface 268, the inner and outer surfaces 267, 268 defining the inner and outer sidewalls of the compression member 260. The inner surface 267 can extend distally beyond the outer surface 268, such that the distal-facing surface 266 can be oriented at an angle to a longitudinal axis L₂ of the compression member 260 and such that the distal-facing surface 266 forms a cone. A distal-most tip of the cone terminates in a sharp edge 261. Similar to the edge 161 of compression member 160 of FIG. 2, the edge 261 in FIG. 3 can form a ring-shaped line contact with the head of the bone anchor to facilitate gripping of the head. The sloped orientation of the distal-facing surface 266 can increase a sharpness of the edge 261 such that the edge 261 is configured to dig into the head and further reduce a risk of slippage of the head of a bone anchor with respect to the receiver member.

Referring now to FIG. 4, in another embodiment a compression member 360 can have a first spherically shaped distal-facing surface 366 and second spherically shaped distal-facing surface 366′ extending distally from the first distal-facing surface 366. The first distal facing surface 366 can have a radius of curvature that is different, e.g., less, than a radius of curvature of the second distal-facing surface 366′, such that a ring-shaped crest 361 is formed at the intersection of the first distal-facing surface 366 and the second distal-facing surface 366′ for gripping the head of a bone anchor. A largest diameter of the crest 361 can be smaller than a diameter of the head of the bone anchor where the crest 361 grips the head, thus creating an interference fit between the compression member 360 and the head. The dimensions of the crest 361 as shown in FIG. 4 are exaggerated for the sake of illustration. While the position of the crest 361 can vary, in the illustrated embodiment the crest is shown at a general mid-portion of the distal-facing surface such that a height of the first distal-facing surface 366 is substantially the same as a height of the second distal-facing surface 366′. This configuration can allow the edge 361 to engage the head of the bone anchor at a desirable location. A person skilled in the art will appreciate, however, that the location of the edge 361 can vary as may be desired based on the size of the head of the bone anchor. Moreover, as with the embodiment of FIG. 2, the edge 361 can extend at any angle relative to the longitudinal axis L₃, including perpendicular or at some other angle.

FIG. 5 illustrates another embodiment of a compression member 460 having an angled distal-facing surface that extends in a plane P₁ that is transverse and non-perpendicular to the longitudinal axis L₄ of the compression member 460. As a result, the compression member 460 can have a first hemicylindrical portion 468A that extends distally beyond a distal-most end of a second hemicylindrical portion 468B such that a first distal-facing surface 466A can be distal and offset along a longitudinal axis L₁ of the compression member 460 from a second distal-facing surface 466B. The angled distal-facing surface can also cause the first distal-facing surface 466A to have a shape that differs a shape of the second distal-facing surface 466B. In particular, the compression member 460 can have a generally spherical recess formed in the distal end thereof, and the spherical recess can have a center point (not shown) that is positioned along the longitudinal axis L₄ of the compression member 460. The plane P₁ can intersect the perimeter of the sphere at a location (shown as edge 461B) that causes the second distal-facing surface 466B to be co-planar with plane P₁. The second distal-facing surface 466B will thus define an edge 461B for creating a semicircular line contact with the head of a bone anchor, whereas the spherical shape of the first distal-facing surface 466A will form a negative of a portion of the head against which the first distal-facing surface 466A abuts. The first distal-facing surface 466A will thus support the head on one end of the head while the edge 461B on the second distal-facing surface 466B can cut into the head at an opposed end of the head. The compression member 460 can thus exert a compressive force on the head that acts at an angle to the longitudinal axis L₄ of the compression member 460 and that can balance an opposing resistive force of the bone anchor against the compression member 460 when the bone anchor is oriented at an angle to the longitudinal axis L₄. As further shown in FIG. 5, the first distal-facing surface 466A can have a radius of curvature corresponding to a radius of curvature of a head of a bone anchor where the first distal-facing surface 466A grips the head, although the radius of curvature of the first distal-facing surface 466A can be smaller than the radius of curvature of the head to create an interference fit with the head. In one embodiment, a largest diameter of an inner cylindrical surface 467 of the compression member 460 can be smaller than a diameter of the head where the compression member 460 grips the head.

In another embodiment, a distal end of a compression member can have multiple gripping features thereon to facilitate gripping a head of a bone anchor. By way of non-limiting example, FIG. 6 illustrates a compression member 560 having a plurality of teeth 563 formed on a distal-facing surface 566. As with previous embodiments, the distal-facing surface 566 can extend between an inner cylindrical surface 567 and an outer cylindrical surface 568, with the inner and outer cylindrical surfaces 567, 568 defining inner and outer sidewalls of the compression member. Similar to the edge 261 described above and illustrated in FIG. 3, sharp crests 561 of the teeth 563 can grip the head of a bone anchor along ring-shaped line contacts to reduce a risk of slippage. The teeth 563 can impart the additional advantage of providing multiple line contacts with the head to provide a stronger grip on the head. Each of the crests 561 can have a radius of curvature, either the same or different from one another, corresponding to the radius of curvature of the head. A largest diameter of at least one of the crests 561 can be slightly smaller than a diameter of the head where the at least one of the crests 561 grips the head to create an interference fit between the compression member 560 and the head.

The teeth 563 can be of any size, shape, and number. In the illustrated embodiment of FIG. 6, the teeth 563 include two planar, substantially perpendicular side walls that meet at the sharp crests 561, although the side walls of the teeth can be oriented at various angles with respect to one another to form sharper or duller crests 561. The teeth 563 can extend along any distance around a circumference of the distal-facing surface 566, although in the illustrated embodiment the teeth 563 extend along the entire circumference of the distal-facing surface 566. Each of the teeth 563 extend in a plane that is substantially perpendicular to a longitudinal axis L₅ of the compression member 560, but the teeth 563 can be oriented in any plane, either the same or different from one another. Moreover, the teeth 563 can be disposed along the distal-facing surface 566 at any distance from one another, although the teeth 563 of the illustrated embodiment are disposed at regular intervals along the longitudinal axis L₅ of the compression member 560. In one aspect, the teeth can be composed of one or more flexible materials, such that the teeth 563 are configured to deform upon contact with the head of a bone anchor. In this embodiment, the teeth 563 can form ring-shaped band contacts with the head, the band contacts having a width measured along the longitudinal axis L₅ of the compression member 560 that corresponds to an amount of deformation of the teeth 563.

FIG. 7 illustrates another embodiment of a compression member 660 having teeth 663 for gripping the head, in which the teeth 663 can each have spherical surfaces 661 such that the teeth 663 can form a plurality of ring-shaped band contacts with the head of a bone anchor. The teeth 663 can be configured similarly to the teeth 553 of the compression member 560 of FIG. 6, however the spherical shape of the spherical surfaces 661 of the teeth 663 can simplify the manufacturing process and can reduce a risk of deformation of the head by the teeth 663. To further reduce a risk of deformation of the head by the teeth 663, the teeth 663 can be comprised of one or more flexible materials, such that the teeth 663 are configured to deform into contact with the head along the ring-shaped band contacts.

FIG. 8 illustrates another embodiment of a compression member 760 having a skirt 769 that extends around the head of the bone anchor and that can improve the structural integrity of the compression member 760. Like compression member 160, compression member 760 can include an edge 761 that is configured to grip the head of the bone anchor. In this embodiment, however, a distal end wall 766 of the compression member 760 can include a first inner portion 762 and a second outer portion 764 that extends at an angle α to the first portion 762. In the illustrated embodiment, the first inner portion 762 is a planar surface that is oriented substantially perpendicularly to the longitudinal axis L₇. The second outer portion 764 is also a planar surface, but it extends at an angle α to the first portion 762, and therefore the longitudinal axis L₇, thereby defining a skirt 769 that can extend around the head of the bone anchor. The second portion 764 can extend distally by any desired distance D₇ from a distal-most end of a U-shaped seat 765 of the compression member 760. The distance D₇ can be large enough to cause the skirt 769 to help resist a distally-directed force applied to the compression member 760 by, e.g., a closure mechanism, which can thereby improve the structural integrity of the compression member 760 and reduce a risk of breakage of the compression member 760. In general, the distance D₇ can be in the range of about 0.7 mm to 4.5 mm.

Notably, compression members 360, 460, 560, 660 also include similar “skirt” portions that extend distally beyond a distal-most portion of U-shaped seats of compression members 360, 460, 560, 660. Like skirt 769, the distally-extending portions of compression members 360, 460, 560, 660 can improve the structural integrity of compression members 360, 460, 560, 660.

Unlike the distally-extending portions of compression members 360, 460, 560, 660, however, the skirt 769 of compression member 760 can be configured not to contact the head of the bone anchor unless the shaft of the bone anchor is moved beyond a certain angle β with respect to a longitudinal axis L₇ of the compression member 760. The skirt 769 can thus function as a stop to prevent movement of the bone anchor beyond the angle β. In particular, the second portion 764 of the distal-facing surface 766 can be configured such that it does not contact the head of the bone anchor until the shaft of the bone anchor is moved beyond an angle β with respect to the longitudinal axis L₇. When the bone anchor is moved such that it is oriented at the angle β with respect to the longitudinal axis L₇, the head of the bone anchor can come into contact with the second portion 764, which can prevent the bone anchor from moving beyond the angle β. Thus, when implanted in a patient, the second portion 764 can help to prevent accidental and/or excessive movement of the bone anchor. As will be appreciated by a person of skill in the art, the skirt 769 can extend around any circumferential length of the head of the bone anchor, for example around an entire circumference of the head, such that the skirt 769 can prevent movement of the bone anchor beyond the angle β in any direction. In another aspect, the skirt 769 can extend around only a portion of the circumference of the head, such that the skirt 769 only inhibits movement of the bone anchor beyond the angle β in certain directions. The angles α, β can vary to provide for any desired amount of movement of the bone anchor within the receiver member, although the angle α is generally in the range of about 120 to 170 degrees and the angle β is generally in the range of about 30 to 60 degrees.

A person skilled in the art will appreciate that all of the aforementioned features for increasing engagement between a compression member and a bone anchor can be provided on a receiver member so as to similarly increase engagement between a receiver member and a bone anchor. The gripping features of the receiver member can be used either in place of or in addition to gripping features formed on a compression member, and can be of any size, shape, and number. FIG. 9 illustrates one example of a receiver member 214 having a distal end 232 with a plurality of teeth 215 configured to grip the head of a bone anchor. The teeth 215, shown in more detail in FIG. 10, can be formed on a distal inner surface 235 of the receiver member 214 that is configured to polyaxially seat the head of a bone anchor, and can be of any size, shape, and number, similar to those discussed above regarding compression members 560 and 660 of FIGS. 6 and 7. The teeth 215 can extend around any portion of the circumference of the distal inner surface 235 and can be positioned at various locations along a length measured along a longitudinal axis L_(R) of the receiver member 214. As shown in FIG. 11, the teeth 215 can cut into the head to substantially prevent movement of the head when the receiver member 214 is locked to the bone anchor. In the illustrated embodiment, the receiver member 214 is a favored-angle receiver having teeth 215 formed only on a first portion 235A of the distal inner surface 235, i.e., on a distal-most end that extends distally beyond a distal-most end of an opposed second portion 235A of the distal inner surface 235. The distal-most end of the second portion 235A can be in the form of an edge that is opposed from the teeth 215 and can thus provide for opposed clamping of the head 18 similar to that described above with respect to the compression member 460 of FIG. 5.

FIGS. 12-16 illustrate exemplary embodiments of a closure mechanism for use with a bone anchor assembly of the type described above. Similar to the closure mechanism 16 explained above, the illustrated closure mechanisms can include inner and outer members—the inner member being configured to secure the spinal rod within the receiver member and the outer member being configured to secure the compression member and the bone anchor within the receiver member. The inner member can be configured to removably mate with the outer member by, e.g., threaded engagement. Each of the closure mechanisms of FIGS. 12-16 can have at least one feature thereon configured to prevent relative movement of the inner and outer members once mated, absent application of a force applied thereto that equals or exceeds a threshold force. Thus, once the inner member has been mated to the outer member, the inner member will not migrate relative to the outer member unless a force equal to or exceeding the threshold force is applied thereto. This can help to reduce a risk of accidental removal of the inner member from the outer member, thus stabilizing the closure mechanism during transport and sterilization. When the bone anchor assembly is implanted in a patient's body, it can help to secure both the bone anchor and the spinal rod within the receiver member at a desired orientation.

In the illustrated embodiment of FIGS. 12 and 13, for example, a closure mechanism 216 includes an inner set screw 272 configured to engage an outer set screw 270 by interference fit. In particular, inner threads 271 of the outer set screw are configured to engage outer threads 273 of the inner set screw 272 such that the inner set screw 272 will not move relative to the outer set screw 270 unless a force equal to or exceeding a threshold force is applied thereto. The interference fit between threads 271 and threads 273 can be created in a variety of ways, although in the exemplary embodiment the interference fit is created by a difference in pitch between at least a portion of threads 271 and threads 273. By way of non-limiting example, a pitch P_(I) of threads 271 on the outer screw 270 can be smaller than a pitch P_(O) of threads 273 on the inner screw 272 along at least a portion thereof. As shown in FIG. 13, the difference in pitch can create regions of overlap 275 between edges of the threads 271, 273 that provide resistance to movement of the inner set screw 272 relative to the outer set screw 270. The pitch difference can be adjusted to customize the threshold force required to overcome the resistance and move the screws 270, 272 relative to one another. For example, a larger pitch difference can increase the threshold force. In general, the pitch P₁ can be in the range of about 0.9 mm to 0.914 mm, the pitch P_(O) can be in the range of about 0.890 mm to 0.899 mm or 0.915 mm to 0.924 mm, and the pitch difference can be in the range of about 0.001 mm to 0.024 mm. Pitch P_(I) and pitch P_(O) can vary along a length of the screws 270, 272, for example such that only a portion of the threads 271, 273 are engaged via interference fit.

As will be appreciated by a person of skill in the art, a number of parameters of the screws 270, 272 can be adjusted to customize the threshold force necessary to move the screws 270, 272 relative to one another. By way of non-limiting example, referring to FIG. 13, a height H_(I) of threads 273 of the inner screw 272 can be above standard clearance levels for use with threads 271 of the outer screw 270 that have a height H_(O). Thus, although a height difference between the height H_(I) of threads 273 and the height H_(O) of threads 271 is not large enough to create areas of overlap, similar to the overlap areas 275 created by the pitch difference, the threads 271, 273 can provide greater interference to avoid de-threading as compared with traditional screw assemblies. In general, the height H_(I) can be in the range of about 0.466 mm to 0.476 mm, the height H_(O) can be in the range of about 0.477 mm to 0.487 mm, and the height difference can be in the range of about 0.001 mm to 0.021 mm

FIGS. 14-16 illustrate another embodiment of a closure mechanism including inner and outer set screws secured together by interference fit. In the embodiment of FIGS. 14-16, an inner set screw 372 of a closure mechanism 316 is mechanically deformed to dig into a portion of an outer set screw 370. During manufacturing, a blunt instrument can be used to deform at least a portion of the threads 373. As shown in FIG. 15, the instrument is inserted between two threads and the deformation caused a portion of an upper thread 373 a to be deformed proximally, and a portion of a lower thread 373 b to be deformed distally. As shown in FIG. 16, only a small, outermost portion of the threads 373 a, 373 b are deformed to create protrusions 377 a, 377 b. Thus, when engaged with the threads 371 of the outer set screw 370, protrusion 377 a can dig into an upper thread 371 a of the outer set screw 370 and protrusion 377 b can dig into a lower thread 371 b of the outer set screw 370 to create overlapping regions 375 a, 375 b. An extent and location of the deformation can be adjusted to customize a threshold force that must be applied to the closure mechanism 316 to move the inner set screw 372 relative to the outer set screw 370 to overcome a frictional force created by the interference fit. For example, creation of the protrusions 377 a, 377 b over a larger portion of the threads 373 a, 373 b can create more friction between the inner and outer screws 372, 370 to increase the threshold force. A person of skill in the art will appreciate that nearly any deformation of the inner set screw 372 and/or the outer set screw 370 can create an interference fit that resists relative movement of the screws 370, 372. Also, deformation of any number and portion of the threads 371, 373 can be performed during manufacturing.

As will be appreciated by a person of skill in the art, interference fits between the inner and outer members can be accomplished in a variety of ways. By way of non-limiting example, at least one of the outer threads of the inner member can have a height that is greater than a height of at least one of the inner threads of the outer member, such that outer tips of the outer threads of the inner member dig into the outer member when the inner and outer members are threadably mated to each other. Additionally or alternatively, at least one of the inner threads of the outer member can have a height that is greater than a height of at least one of the outer threads of the inner member. It will further be appreciated by a person of skill in the art that any of the aforementioned means of increasing the threshold force required to move the inner and outer members relative to each other can be used in conjunction with one another.

In use, a bone anchor assembly can be assembled, either during manufacturing, prior to use, or intraoperatively, by passing an elongate shank of a bone anchor in a proximal-to-distal direction through an aperture formed in a distal end of a receiver member. In other embodiments, bottom-loading bone anchors can be utilized. A proximal head portion of the bone anchor can be polyaxially seated within the polyaxial recess in the receiver member. A compression member can be inserted between the opposed arms of the receiver member, proximal to the proximal head of the bone anchor. An angle of the bone anchor with respect to the receiver member can be adjusted to a desired angle. In an exemplary embodiment, the compression member is configured to apply a frictional force to the bone anchor to maintain the bone anchor in a desired angular orientation prior to lock, while still allowing a force to be applied to the bone anchor to move the bone anchor relative to the receiver member.

During an implantation procedure, the bone can be prepared to receive the bone anchor, e.g., by drilling an appropriately sized hole. A driver tool can be fitted with the bone anchor to drive the bone anchor into the prepared hole in the bone. A spinal fixation element, e.g., a rod, can be located in between the arms of the receiver member, and a closure mechanism can be applied to the receiver member, proximally of the rod, to urge the spinal rod and the compression member distally such that a distal-facing surface of the compression member comes into contact with the head of the bone anchor. Gripping features of the compression member can grip the head of the bone anchor to grip the head portion of the bone anchor with greater friction as compared with a distal end formed as a negative of the head portion of the bone anchor on the head of the bone anchor. In particular, the compression member can make at least one of line contact and band contact with the head portion of the bone anchor. As a result, the bone anchor is locked at the desired angle with respect to the receiver member. Similarly, or alternatively, gripping features on the receiver member can have the same effect to lock the receiver member in a fixed position relative to the bone anchor. Such gripping features on the compression member and/or the receiver member can provide increased contact and further prevent the risk of slippage as compared to compression members and receivers lacking such features.

Where a bone anchor assembly includes both a favored-angle receiver member, such as receiver 214 shown in FIG. 9, and a favored-angle compression member, such as compression member 460 shown in FIG. 5, the angled distal-surfaces can be oriented relative to one another so as to direct the compressive forces in a desired direction. For example, as shown in FIG. 17, the distal-facing surface of compression member 460 extends in a plane P_(C) that is transverse and non-perpendicular to a plane P_(R) of the distal-facing surface of the receiver member 214. As a result, a compressive force F_(C) exerted by the compression member 460 can act on one side of the head 18 of the bone anchor in a direction that is opposite to a compressive force F_(R) exerted by the receiver member 214 on the opposite side of the head 18. Both compressive forces F_(C), F_(R) can act at an angle to the longitudinal axis L₄ of the compression member 460 and can counterbalance one another to stabilize the bone anchor 12. Conversely, in FIG. 18, the plane P_(C) of the distal-facing surface of the compression member 460 is oriented substantially parallel to the plane P_(R) of the distal-facing surface of the receiver member 214. In this configuration, the compressive forces F_(C), F_(R) exerted by the compression member 460 and the receiver member 214 can act on a same side of the head 18 but in opposite proximal-distal directions. This alignment of compressive forces F_(C), F_(R) can similarly help to stabilize the head 18, particularly where the bone anchor 12 is oriented at an angle to the longitudinal axis L₄ of the compression member 460 in a direction X as shown in FIG. 18. A person skilled in the art will appreciate that the planes P_(C), P_(R) of the distal-facing surfaces of the compression member 460 and receiver member 214 can be oriented in a desired configuration during manufacturing, e.g., via a swaging process described above, or before or during surgery.

As mentioned above, the compression member can be configured to directly transmit a distally-directed force applied thereto by the closure mechanism to thereby lock the bone anchor in a fixed position. However, the compression member can alternatively be configured to absorb at least a portion of the force applied thereto by the closure mechanism, particularly where the force exceeds a threshold force, such that some of the force applied to a proximal end of the compression member is not transferred to the bone anchor. This can help to ensure that neither the compression member nor the bone anchor is damaged by the application of excessive force thereto. By way of non-limiting example, at least a proximal portion of the compression member can be configured to compress inwardly or expand outwardly upon the application of a distally-directed force exceeding the threshold force. In one exemplary embodiment, opposed arms of the compression member can each have proximal end surfaces that are configured to seat a distal end surface of the closure mechanism such that a distally-directed force applied by the closure mechanism to the compression member can cause the opposed arms to move inwardly toward one another when the force exceeds the threshold force.

A proximal portion of the compression member and a distal portion of the closure mechanism can be configured in a variety of ways to cause the opposed arms of the compression member to compress inwardly. By way of non-limiting example, illustrated using the compression member 760 of FIG. 8 and the outer set screw 370 of FIG. 14, the proximal portion of the compression member 760 can have a concave shape that corresponds to a convex shape of the distal portion of the screw 370. As shown in FIG. 19, proximal end surfaces 764A, 764B of the compression member 760 can each have a convex shape, and the compression member 760 can have a height that decreases radially outward from an inner wall 767 to an outer wall 768. Conversely, the screw 370 can have a convex shape and, to correspond to the shape of the compression member 760, the outer wall of the screw 370 can extend distally beyond the inner wall of the screw 370. The corresponding shapes can allow for both the compression member 760 and the screw 370 to rotate freely relative to one another when mated together, while at the same time preventing inward and outward movement of the components relative to one another. When a distally-directed force that exceeds the threshold force is applied to the compression member 760 by the screw 370, the outer portion of the screw 370 can exert a lateral force on the arms 762A, 762B to thereby push the arms 762A, 762B of compression member 760 toward one another. As will be appreciated by a person of skill in the art, the opposed arms 762A, 762B can be formed from a material that is sufficiently flexible to allow the opposed arms 762A, 762B to move toward one another when acted upon by a force exceeding the threshold force, but sufficiently rigid to maintain a fixed distance from one another when a force applied thereto is below the threshold force. The arms 762A, 762B can be formed from either the same or different materials than the remainder of the compression member 760.

As will also be appreciated by a person of skill in the art, any number of force-absorbing mechanisms, e.g., a spring, can be used to absorb additional force beyond the threshold force to prevent failure of and/or damage to the bone anchor assembly. For example, as mentioned above, the compression member 760 can have a skirt 769 that can resist some of the force applied to the compression member 760 by a closure mechanism, thus helping to preserve the structural integrity of the compression member 760 under large forces.

Although the invention has been described by reference to specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims. 

What is claimed is:
 1. A bone anchor assembly, comprising: a bone anchor having a proximal head portion and a distal shank portion; a receiver member having a polyaxial seat formed therein for polyaxially seating the head portion of the bone anchor; a closure mechanism matable with the receiver member to lock the bone anchor within the receiver member; and a compression member configured to be disposed within the receiver member between the bone anchor and the closure mechanism, the compression member having a proximal end configured to seat a distal end of the closure mechanism, the proximal end having a shape that causes the compression member to move radially inward when a distally-directed force is applied thereto by the closure mechanism that exceeds a threshold force.
 2. The bone anchor assembly of claim 1, wherein the closure mechanism comprises at least one threaded member.
 3. The bone anchor assembly of claim 1, wherein, when the distal end of the closure mechanism is seated in the proximal end of the compression member, the compression member is freely rotatable but does not move radially inward and outward unless a force greater than the threshold force is applied thereto.
 4. The bone anchor assembly of claim 1, wherein the proximal end of the compression member has a convex shape that corresponds to a concave shape of the distal end of the closure mechanism.
 5. The bone anchor assembly of claim 1, wherein the proximal end of the compression member has a height that decreases radially outward.
 6. The bone anchor assembly of claim 1, wherein the compression member has a distal end that abuts against the proximal head of the bone anchor, the distal end including a distally-extending skirt formed around at least a portion of an outer perimeter thereof such that the skirt extends distally beyond a proximal-most end of the head portion of the bone anchor when the compression member and the bone anchor are disposed within the receiver member.
 7. The bone anchor assembly of claim 6, wherein the skirt extends all the way around a circumference of the head portion of the bone anchor.
 8. The bone anchor assembly of claim 6, wherein an inner portion of a distal facing end surface of the compression cap is planar and extends substantially perpendicularly to a central longitudinal axis of the compression cap, and an outer portion of the distal facing end surface of the compression cap extends at an angle relative to the inner portion to define the skirt.
 9. The bone anchor assembly of claim 1, further comprising an expandable clip seated within a groove formed in the receiver member, the expandable clip being configured to apply a frictional drag force to the head portion of the bone anchor.
 10. A bone anchor assembly, comprising: a bone anchor having a proximal head portion and a distal shank portion; a receiver member having a polyaxial seat formed in a distal portion thereof for polyaxially seating the head portion of the bone anchor; and a closure mechanism having an outer member having outer threads configured to threadably mate with threads formed in the receiver member to thereby lock the proximal head of the bone anchor with respect to the receiver member, and having a central opening with inner threads formed therein, and an inner member having outer threads formed therearound for threadably mating with the inner threads formed in the outer member, the inner member being configured to lock a spinal fixation element within the receiver member, wherein at least one of the outer threads on the inner member and the inner threads on the outer member are configured to provide an interference fit when the inner member is threadably mated to the outer member.
 11. The bone anchor assembly of claim 10, wherein at least a portion of the outer threads on the inner member have a pitch that is different from a pitch of the inner threads on the outer member.
 12. The bone anchor assembly of claim 10, wherein at least one of the outer threads on the inner member and the inner threads on the outer member includes a mechanical deformation formed thereon and configured to create the interference fit when the inner and outer members are threadably mated.
 13. The bone anchor assembly of claim 12, wherein the mechanical deformation comprises a surface protrusion formed on at least one of the outer threads on the inner member and the inner threads on the outer member and configured to extend into a surface of an adjacent thread.
 14. The bone anchor assembly of claim 10, further comprising an expandable clip seated within a groove formed in the receiver member, the expandable clip being configured to apply a frictional drag force to the head portion of the bone anchor.
 15. The bone anchor assembly of claim 10, further comprising a compression member configured to be disposed within the receiver member between the bone. 